The steering torque introduced into the steering wheel by the driver is transmitted via the steering spindle to the steering gear, by which track rods are moved and, as a result, a steering angle of the steerable wheels of a motor vehicle is brought about. The rotatable mounting of the steering spindle takes place in a sleeve unit which can be designed as a guide box or steering column tube, depending on the structural form. The sleeve unit is fitted to the vehicle body via a supporting unit, for example a bracket unit.
In order to improve the occupant's safety in the event of a vehicle collision or, in other terms, a crash, a crash device or a crash system is provided in a steering column. A crash device is realized by the sleeve unit being pushable forwards in the longitudinal direction relative to the body-mounted supporting unit when the body of the driver impacts against the steering wheel during a head-on collision at high speed, with a large force being exerted on the steering wheel, said force exceeding a limit value which occurs only in the event of a crash. In order to ensure controlled braking here of the body impacting against the steering wheel, an energy absorption device is coupled between the sleeve unit and the supporting unit, which are customarily fixed to each other by the clamping device of a position adjustment device during normal operation, but are pushed together relative to each other in the event of a crash. Said energy absorption device converts the introduced kinetic energy into plastic deformation of an energy absorption element, in the design of the type in question by bending an elongate bending element, for example a bending wire or bending strip.
A bending wire or bending strip, as is described, for example, in the case of a steering column of the type in question in DE 10 2008 034 807 B3, has a substantially U-shaped basic shape with a first limb—referred to as the input limb—which merges via a bend of 180° into a second limb—called the fixed limb. The input limb extends in the longitudinal direction and can be connected at its free end to the sleeve unit. The fixed limb can be fixed to the supporting unit with a fastening portion which is likewise located in the end region of the limb. Alternatively, it is possible for the input limb to be connected to the supporting unit and for the fixed limb to be connected to the sleeve unit.
During normal operation, supporting unit and sleeve unit are connected fixedly to each other via the bending element and are thereby positioned relative to each other in the longitudinal direction. In the event of a crash, a large force acts in the longitudinal direction between sleeve unit and supporting unit, as a result of which the bending element deforms and kinetic energy is absorbed. The deformation takes place specifically in that the input limb, which is moved by the sleeve unit, is displaced in the longitudinal direction parallel to the stationary fixed limb, and therefore the position of the bend with respect to the free end of the input limb migrates or rolls with a bending radius remaining substantially constant, that is to say, the input limb is bent within the plane provided by the U-shaped basic shape in the bending direction of the bend and peters out behind the bend into the fixed limb. It is advantageous here that the quantity of energy which is converted or absorbed as the bend progresses can be defined relatively precisely by the bent material cross section of the bending element.
During the bending of the input limb, transverse forces act between the limbs of the bending element transversely with respect to the longitudinal axis and have to be supported so that the radius of the bend, progressing in the event of a crash, remains substantially constant, and the limbs are not deflected laterally, and the bending radius of the bend is not expanded in an undefined manner. For this purpose, it is known from the abovementioned DE 10 2008 034 807 B3 to arrange the bending element between lateral boundary walls extending in the longitudinal direction. As a result, lateral deflection during the deformation of the limbs in the event of a crash is effectively prevented and therefore a uniformly defined force profile during the deformation of the bending element is ensured.
However, it is problematic that, in the event of a crash, at least one of the limbs slides with its outer side facing away from the bend along the boundary wall in the longitudinal direction. The frictional forces occurring in the process can interfere with the uniform force profile during the energy absorption in the event of a crash. To counteract this, it has indeed been proposed to provide at least the outer side of the bending element with a friction-reducing coating. However, this requires the use of special coating material and in particular an additional processing step, as a result of which the manufacturing outlay and the costs are increased.
Thus a need exists for a steering column with an improved energy absorption device, and an energy absorption device that permits simplified production and reliable functioning in the event of a crash.